Despite the importance of political knowledge to democracy, there is still only a limited understanding of the political knowledge of the electorate. While research and empirics generally point to the existence of a positive association between partisanship and political knowledge, most of the electorate fail to notice the probable negative influence of party identification on political knowledge. In this paper, in accordance with the theory of motivated reasoning, we claim that partisanship plays a role in sifting and selecting political information, and in deleting, ignoring, and disbelieving the information against personal prior beliefs. Moreover, the electorate might even distort the information in order to sustain their political decisions. Therefore, as the voters’ party identification becomes stronger, the influence of political information on political knowledge becomes insignificant. We further test our motivated reasoning hypotheses against the TEDS 2012 data. The result supports our hypotheses and shows that, as the voters’ party identification increases, the marginal effect of political information on political knowledge decreases.